1. Field of the Invention
The present invention relates to a non-aqueous adhesive composition for a polarizing plate to bond a polarizer and a protective film or a compensation film, and a polarizing plate prepared through bonding by using the non-aqueous adhesive composition.
2. Description of the Related Art
A liquid crystal display device has been used in various display devices due to having characteristics such as low power consumption, low-voltage operation, lightweightness, and thinness. The liquid crystal display device may be composed of many materials such as a liquid cell, a polarizing plate, a retardation film, a light collection sheet, a diffusion film, a light guide plate, and a light reflecting sheet. Therefore, improvements targeting enhancements of productivity, lightening, and brightness have been actively undertaken by reducing the number of constituent films, or decreasing the thicknesses thereof.
In general, a polarizing plate has been used in a structure, in which a protective film is stacked on one or both sides of a polarizer formed of a polyvinyl alcohol (hereinafter, referred to as “PVA”)-based resin dyed with a dichroic dye or iodine by using a water-based adhesive prepared by dissolving a PVA-based material in water. However, this may be inconvenient, in that a corona treatment or a primer treatment may be performed due to low adhesive strength between the film and the water-based adhesive.
The protective film may be formed of triacetyl cellulose (TAC), polyethylene terephthalate (PET), a cycloolefin polymer (COP), an acrylic film, or a combination thereof.
An acrylic adhesive, an adhesive for a dry laminate in which a polyurethane-based resin solution and a polyisocyanate resin solution are mixed, a styrene butadiene rubber-based adhesive, an epoxy-based adhesive, a PVA-based adhesive, a polyurethane-based adhesive, an adhesive containing a polyester-based ionomer type urethane resin and a compound having a glycidyloxy group, or a thermocurable-type adhesive are known as adhesives which may be used for bonding the polarizer and the protective film. In particular, a water-based adhesive formed of an aqueous solution having a PVA-based resin is used in most cases.
However, when the protective film is an acrylic film, adhesive strength, with respect to the water-based adhesive may be low, due to the characteristics thereof. As a result, there may be limitations in reprocessability or cuttability. For this reason, there may be inconvenience, in that the protective film may need to be coated with a primer and then bonded with the water-based adhesive, or wetting, with respect to water, is increased by applying corona discharge.
Meanwhile, in addition to the degradation of adhesive strength according to a substrate, when different substrates are used as protective films on one side and the other side of a PVA device, there may be problems like the occurrence of curling in a polarizing plate caused by a drying process of the water-based adhesive and a degradation of initial optical physical properties, and therefore, a non-aqueous based adhesive was developed in order to resolve the foregoing problems.
In general, with respect to the non-aqueous based adhesive, the thickness of a final adhesive layer tends to be thicker in comparison to the case of the water-based adhesive. Therefore, the thickness of the adhesive layer may be decreased by minimizing the viscosity thereof. At this time, water resistivity becomes poor when the content of an oligomer is reduced and a hydrophilic monomer is used in order to maintain the non-aqueous based adhesive in a state of low viscosity, and a decrease in adhesive strength generally occurs when a polyfunctional monomer or a hydrophobic monomer is added in order to improve water resistance. As another method of increasing water resistance, an isocyanate-based compound having good water resistivity may be added, but there may be a limitation in solution stability caused by the hydrophilic monomer having a reaction therewith.
Therefore, as an adhesive composition which may be used in various substrates, e.g., a TAC film, an acrylic film, a COP film, or a PET film, the development of a non-aqueous adhesive for a polarizing plate having excellent adhesive strength as well as improved water resistivity and degree of curing, and particularly excellent thermal shock properties at low viscosity, and a polarizing plate bonded by using the non-aqueous adhesive are required.